In the art of steelmaking, oxygen lance assemblies have long been used to increase the productive capacity of electric arc furnaces or, simply, "electric furnaces" through the introduction of oxygen at high velocity directly against the molten bath. The nozzle structure of such a lance assembly oftentimes includes an oxygen nozzle and a nozzle for introducing a flow of fine-grain solid material in a form of a metallurgical agent, such as carbon, lime, and/or other metallurgical agents of many forms, for further enhancement of the steelmaking process dependent on desired results.
One objective in attempting to improve the performance of an electric furnace is to cause an increase in the volume of the slag that normally forms on the surface of the molten steel bath. A high slag volume increases the thermal efficiency of the electric furnace in that the slag functions as an insulating layer that preserves the temperature of the bath. Moreover, the slag combines with and removes impurities from the bath. For these reasons, it is desirable to provide a lance assembly capable of delivering oxygen and carbon or other fine-grain solids in a manner to increase volume of the slag and to attain beneficial thermal and chemical reactions for increasing productive capacity.
Oxygen blowing lances have been used in connection with electric furnace steelmaking vessels both to perform their primary function of refining/decarburizing of the metal bath and to provide an additional source of energy to augment "cutting" or melting of the scrap charge by the vessels' electrodes.
When using a conventional straight-barreled oxygen blowing lance in an electric furnace steelmaking vessel, the lance is inserted through a lance opening provided in the sidewall of the vessel. Typically, the lance is inclined and the angle of insertion of the lance ranges from about 20 to 40 degrees with respect to horizontal. In operation, oxygen is injected from the lance immediately after the vessel has been provided with the first bucket charge of scrap and after the lance opening has been cleared of scrap. Although the oxygen impingement upon the cold scrap produces little or no meaningful thermochemical reaction between the oxygen flow and the scrap, it frequently results in secondary combustion at the furnace roof and duct areas, thereby causing excessive exhaust temperatures and decreased service life of the vessel components at these sites. After charging with the second and third buckets of scrap, however, there is sufficient latent heat within the vessel to provide oxygen ignition temperatures within the mass of scrap. Nevertheless, scrap cutting using a conventional oxygen blowing lance as a supplemental energy source remains slow and inefficient.
To supplement the preheating and scrap cutting effect of an oxygen lance in an electric furnace, it has been proposed to insert one or more oxygen/fuel burner assemblies, along with the lance, into the steelmaking vessel. Because of the bulk of equipment, however, such an arrangement necessitates operation through an opened slag removal door. Consequently, the available preheating/scrap cutting area is limited and air entrainment occurs within the vessel which is detrimental to the production of low nitrogen steel grades.
U.S. Pat. No. 4,653,730 discloses a system that incorporates a fuel burner into an oxygen blowing lance. The lance is of arcuate design and is introduced through a lance opening in the furnace sidewall. Because if its arcuate shape, the lance travels in an arcuate path, thereby limiting the distance from the furnace sidewall that the lance can be inserted into the furnace to considerably less than the radius of curvature of the lance and well short of the central regions of the vessel. Accordingly, the operating area of the lance is limited to an area close to the sidewall of the furnace vessel. This localized operating area can result in over-oxidation of scrap and furnace sidewall erosion in the vicinity of the lance. Moreover, to achieve any meaningful distribution of lance-provided energy throughout the scrap charge, U.S. Pat. No. 4,653,730 suggests that a plurality of such lances (e.g., three) be disposed about the circumference of the furnace vessel, thereby multiplying the acquisition and operation costs associated with lance usage.
U.S. Pat. Nos. 3,620,455, 3,823,929, Re. 28,769, 3,827,632, 3,912,243 and 4,047,936 describe lance apparatus including straight-barreled lances having combined fuel burning and oxygen blowing capability. These assemblies are generally effective for their prescribed functions. However, none of these lances are operable to provide, in addition to their oxygen blowing and scrap melting functions, the selective introduction of metallurgical agents such as particulate carbon to enhance the effectiveness of the refining process and ultimate quality of the steel products. U.S. Pat. No. 4,083,540 provides for a oxygen/fuel burner lance assembly without a capability to deliver metallurgical agents during refining. U.S. Pat. No. 4,434,005 describes a steelmaking lance having post-combustion and carbon injection functions but without a fuel burner capability.
An advantage exists, therefore, for a single combined oxygen blowing/fuel burner lance assembly capable of providing the benefits of rapid and generalized preheating and cutting of the scrap charge within an electric furnace while avoiding the diseconomies of multiple lances and the potential harm that may be caused to the scrap and the vessel in the vicinity of the vessel sidewall.
A further advantage exists for such a lance assembly which is additionally capable of introducing, at selected times, a metallurgical agent such as fine-grained carbon or the like, for promoting slag formation and desired end-point chemistry qualities in the molten bath.